1. Field of the Invention
The present invention relates to vacuum pumping devices capable of establishing and maintaining an appropriate vacuum in an enclosure.
2. Description of the Prior Art
Establishing a vacuum in an enclosure is standard practice in industrial processes such as semiconductor fabrication, certain fabrication steps having to be executed in a vacuum.
In such processes, semiconductor substrates are placed in a loading chamber (load lock) connected to a vacuum pumping device that reduces the pressure inside the loading chamber to a value satisfactory for then transferring the semiconductor substrates into a process chamber in which the appropriate fabrication vacuum has been established.
Obviously, each time substrates are loaded or unloaded the gas pressure in the loading chamber must be lowered and then raised, in an alternating manner, which implies the frequent intervention of the vacuum pumping device.
Equally obviously, establishing the vacuum in the loading chamber is not instantaneous, and this constitutes a limit on the overall speed of the fabrication process.
The importance of this limit is in direct proportion to the size of the substrate, especially in the fabrication of flat television or display screens, the loading chamber necessarily having a volume appropriate for containing one or more flat screens.
For example, the loading chambers used for the fabrication of flat screens at present have large volumes, of the order of 500 to 1000 liters, which must therefore be pumped as quickly as possible.
The solution currently used for rapidly pumping these large loading chambers is to use large pumps fitted with large motors. As a result, the pumps and the motors are large and costly and have a high power consumption.
In practice, prior art pumping devices generally comprise at least one primary pump at the discharge end of the vacuum line and at least one secondary pump at the suction end of the vacuum line, i.e. at the outlet from the enclosure to be pumped. The primary pump and the secondary pump are connected in series in the flow path of the pumped gases and are driven by two electric motors that are supplied with power to rotate at nominal speed.
Vacuum pumping devices comprising at least one primary pump and one secondary pump connected in series and driven by two electric motors controlled by electronic control means are also known, for example from the document U.S. Pat. No. 4,699,570. The primary pump is driven at constant speed and its power supply can be interrupted at the end of pumping. The secondary pump is driven at a speed that increases regularly as the pressure in the enclosure is reduced, to reduce the power consumption of the pump.
The improvement in power consumption achieved by a device of the above kind is nevertheless insufficient, in particular for frequent pumping of large enclosures, and the invention aims to propose means that further reduce power consumption in order to render the device applicable to the pumping of large chambers.